I've thought about this a bit, and here are my thoughts.
1) Yes, runDisney should provide a mechanism to allow those who can no longer race to transfer a bib.
2) Though there are a lot of people looking to sell bibs (at least anecdotally), the demand for Disney races is far far higher than the supply. I respect the anecdote about people trying to sell bibs on the princess facebook page... but how many was it really? Even 100 people is a spit in the bucket of the overall registered base.
3) It should not be allowed as a source of profit to the person selling their bib, in fact, it should probably be at least a minor financial loss. Otherwise we'll see a ticketmaster situation where everything is sold out in five minutes and the only avenue to purchase is through scalpers.
4) In particular, I support private-party "transfer" remaining disallowed, due to the above.
With all that said, here's my suggestion:
After races sell out, Disney allows you to add your name to a waitlist (and requires you to provide your payment information at that time). You can remove yourself from the wait list at any time, and in doing so avoid any risk of being charged. (but in doing so, should you rejoin the wait list later, you'll be in the back of the line) The wait list, of course, is in first-come-first-served order.
Disney's policies still should explicitly disallow transfer of bibs privately, whether it be for cost or otherwise. However, they should be offering an option to cancel your registration, which then makes it available to fulfill waitlist requests. The amount of your refund should scale down in time, perhaps from a maximum of 90% initially down to no refund whatsoever at the 60 day mark. The idea here is that there's at least a small financial penalty involved, so that nobody will snatch up registrations just for the purpose of hoarding. A sliding scale refund system encourages those who won't be able to make it to declare that fact sooner rather than later.
I probably haven't thought everything through perfectly, but that's my general idea.